Photovoltaic solar cells are the most recently discovered new method of producing energy, dating from the 1950's Soviet and US satellite power systems. Photovoltaic solar cells produce electricity with very low environmental impact, and are because of this desired by the public. The problem with present photovoltaic solar cells is that they do not produce enough energy for their cost and/or surface area to make them economically viable.
Therefore many technologies have been suggested to improve the efficiency of solar cells. EP 1724841 A1 describes a multilayer solar cell, wherein plural solar cell modules are incorporated and integrally laminated, so that different sensitivity wavelength bands are so that the shorter the centre wavelength in the sensitivity wavelength band is, the more near the module is located to the incidental side of sunlight. This document is cited here as reference. It is currently not known, which are all the factors that cause a shortcoming in the efficiency of the multilayer solar cell. However, based on the studies of the applicant, the general tandem solar cell is hampered the most by the photon-phonon processes that take place outside the band of maximum quantum efficiency of the solar cell, i.e. this is where in frequency space the cell generates a lot of heat. Individual factors pertaining to the disadvantages are also listed in the columns 1 and 2 of U.S. Pat. No. 6,689,949, which is cited here as reference. U.S. Pat. No. 6,689,949 discloses a photovoltaic reflective cavity with several solar cells in the cavity. The solar cells inside the cavity are under filters that filter the light so that the incoming photon flux is more appropriate for the quantum efficiency of the solar cell, i.e. more appropriate for its response or detector response.
NASA and JPL (Jet Propulsion Laboratory) have also proposed an alternative technique, called “Rainbow” where beam splitters and concentrators are used to split the solar spectrum into different bands and focus the different bands of light to different discrete solar cells that can handle the splitted and focused spectrum the best. This scheme requires a very complicated optical arrangement, and has not materialised to anything practical so far.
U.S. Pat. No. 5,021,100 discloses a tandem solar cell that has a reflective film between the first and the second solar cell (incident cell on sunlight side in this publication), where the reflective film is supposed to reflect high energy photons to the second solar cell and let low energy photons to the first solar cell (behind the second solar cell in this publication). This document is cited here as reference. U.S. Pat. No. 5,021,100 has a serious problem in that the reflective film is a bidirectional, i.e. any reflected photons in the first solar cell will leak back to the second solar cell through the reflective film, and cause phonons and heat, as these photons cannot get absorbed in the second solar cell.